1. Field of the Invention
This invention relates to a propellant processing technique. More particularly, the invention relates to a continuous manufacturing process of composite gun propellant using a twin-screw extruder.
2. Technology Background
Gun propellants are basically divided into homogeneous and composite formulations. The homogeneous propellants include single, double, and triple base propellants. Single base propellants are basically nitrocellulose with some ballistic modifiers and stabilizing additives. Double base propellants add nitroglycerine to the nitrocellulose propellant, and triple base propellants further add nitroguanidine. Composite gun propellants offer a broader range of processing characteristics and ballistic parameters. High energy coupled with flame temperature modification provides a broad range of performance characteristics. The binder and plasticizer used has an effect on the susceptibility of the propellant to accidental ignition and the particle size of the oxidizer influences the response of the propellant to unplanned stimuli. For some applications, high energy requirements may override the temperature and vulnerability considerations, thus achieving enhanced performance with accepted risks in propellant hazard or increased barrel wear.
A continuing objective in the design of gun propellants is to provide a gun propellant which is energetic when deliberately ignited, but which exhibits high resistance to accidental ignition from heat, flame, impact, friction, and chemical action. This is especially important in confined quarters such as inside tanks, ships or the like. Propellants possessing such resistance to accidental ignition are known as "low vulnerability ammunition" (LOVA) gun propellants.
Conventional LOVA gun propellants comprise an elastomeric binder, throughout which are dispersed particulates of high-energy material, particularly oxidizers. The two most common oxidizer particulates are RDX (1,3,5-trinitro-1,3,5-triaza-cyclohexane) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclooctane). Mixtures of these oxidizers may be used.
Another type of LOVA propellant has a binder of cellulose acetate or a cellulose acetate derivative. An example of this type of propellant is described in U.S. Pat. No. 4,570,540, the teachings of which are incorporated herein by reference. These types of LOVA propellants are batch processed using a solvent, which entails relatively long processing times and a large number of steps.
In a typical LOVA gun propellant batch manufacturing process, RDX is dried in a twin-cone blender under vacuum to remove the water and alcohol used to desensitize the RDX during shipping. The RDX is then ground on a fluid energy mill to a weight-mean-diameter of less than 5 microns. The RDX is weighed into a batch size increment for mixing. The other LOVA ingredients include cellulose acetate butyrate (CAB), nitrocellulose (NC), ethyl centralite (EC), a liquid coupling agent, and an energetic plasticizer (EP). The ingredients are all added to a horizontal, sigma blade mixer that has been modified to eliminate seals around the blade shafts. Vertical mixers are precluded from this process because the very high viscosity results in inadequate mixing capability. The ingredients are wet with a mixed ethyl acetate/ethyl alcohol solvent having a solvent ratio of about 76% ethyl acetate to 24% ethyl alcohol. The materials are mixed for several hours to assure that the organic binder materials are dissolved and coated onto the RDX. The temperature of the mixer is controlled during this entire cycle so that the solvent mixture is not removed prematurely. When the mix cycle reaches a proper time, determined by the amount of mix energy introduced into the propellant, a vacuum is applied and the solvent level is reduced over a period of time to the proper operating level.
The mix is then dumped and transferred to the blocking and straining area. Approximately 60 pounds of LOVA is put into a die and pressed into a cylinder approximately 12 inches in diameter and 16 inches long. The block is placed in a ram extruder and pressed through a sieve plate to put additional work into the propellant to improve mixing. The spaghetti-like strands are collected and re-pressed in the die to a 60 pound cylinder. The cylinder is transferred to a large ram press with 30 dies. Each die is approximately 0.33 inch in diameter with a 19 perf pin plate to make a perforated grain for the gun propellant. The 60 pound block is extruded in a vertical plane with each strand being collected in a spiral around a cone beneath the die. As the strands exit the dies, the weight of the strands causes an elongation of the strands and a necking down of the diameter. This produces a variable diameter strand that affects the reproducibility of the grains. The solvent content is approximately 10% during extrusion.
The flexible strands are fed to a rotating blade cutter and cut into pellets approximately 0.5 inches long. The pellets are collected, dried, glazed with graphite to prevent static charges and improve packing, and stored for several weeks to "age" the propellant before it is ballistically accepted. This batch process is costly and very labor intensive. Moreover, the efficiency of the batch mixer produces less than ideal homogeneity and performance reproducibility.
From the foregoing, it will be appreciated that there is a need in the art for continuous composite gun propellant manufacturing processes capable of producing high quality, low cost composite gun propellant.
Such composite gun propellant manufacturing processes are disclosed and claimed herein.